1. Technical Field
The present invention generally relates to a method for improving fluorocarbon elastomer seal compatibility.
2. Description of the Related Art
Lubricating oil compositions used to lubricate internal combustion engines and transmissions contain a major amount of a base oil of lubricating viscosity, or a mixture of such oils, and one or more lubricating oil additives to improve the performance characteristics of the oil. For example, lubricating oil additives are used to improve detergency, to reduce engine wear, to provide stability against heat and oxidation, to reduce oil consumption, to inhibit corrosion, to act as a dispersant, and to reduce friction loss. Some additives provide multiple benefits such as, for example dispersant-viscosity modifiers.
Among the most important additives are dispersants which, as their name indicates, are used to provide engine cleanliness and to keep, for example, carbonate residues, carboxylate residues, carbonyl residues, soot, etc., in suspension. The most widely used dispersants today are products of the reaction of succinic anhydrides substituted in the alpha position by an alkyl chain of polyisobutylene (PIBSA) with a polyalkylene amine, optionally post-treated with a boron derivative, ethylene carbonate or other post-treatment reagents known in the specialized literature.
Among the polyamines used, polyalkylene-amines are preferred, such as diethylene triamine (DETA), triethylene tetramine (TETA), tetraethylene pentamine (TEPA), pentaethylene hexamine (PEHA) and heavier poly-alkylene-amines (HPA).
These polyalkylene amines react with the succinic anhydrides substituted by alkyl groups of polyisobutylene (PIBSA) to produce, according to the molar ratio of these two reagents, mono-succinimides, bis-succinimides or mixtures of mono- and bis-succinimides.
Such reaction products, optionally post-treated, generally have a non-zero basic nitrogen content of the order of 5 to 50, as measured by the total base number or TBN, expressed as mg of KOH per gram of sample, which enables them to protect the metallic parts of an engine while in service from corrosion by acidic components originating from the oxidation of the lubricating oil or the fuel, while keeping the said oxidation products dispersed in the lubricating oil to prevent their agglomeration and their deposition onto metal parts.
Dispersants of mono-succinimide or bis-succinimide type are even more effective if their relative basic nitrogen content is high, i.e. in so far as the number of nitrogen atoms of the polyamine is larger than the number of succinic anhydride groups substituted by a polyisobutenyl group.
However, the higher the basic nitrogen content of these dispersants, the more they favor the attack of the fluorocarbon elastomer seals used in modern engines, because the basic nitrogen tends to react with the acidic hydrogen atoms of this type of seal, and this attack results in the formation of cracks in the elastomer surface and the loss of other physical properties sought in this type of material.
U.S. Pat. No. 6,124,247 (“the '247 patent”) discloses that dispersants of mono-succinimides or bis-succinimides are even more effective if their relative basic nitrogen content is high, i.e., insofar as the number of nitrogen atoms of the polyamine is larger than the number of succinic anhydride groups substituted by a polyisobutenyl group. However, the higher the basic nitrogen content of these dispersants, the more they favor the attack of the fluoroelastomer seal used in modern engines, because the basic nitrogen tends to reach with the acidic hydrogen atoms of this type of seal, and this attack results in the formation of cracks in the elastomer surface and the loss of other physical properties sought in this type of material. The '247 patent further discloses that by using lubricating oil compositions containing a dispersant of mono-succinimide or bis-succinimide type, post-treated or not, in combination with a borated glycerol ester, one obtains a composition compatible with fluorocarbon elastomers.
Accordingly, it would be desirable to develop lubricating oil compositions which exhibit improved fluorocarbon elastomer seal compatibility.